P
pawihte
- Jan 1, 1970
- 0
I tried my hand at making a 9V power supply with an MC34063A. I
get the correct DC voltage output but was disappointed with the
very dirty output. I know that, in general, simple switched-mode
PSes have poorer performance than linear types, but what I
observed was worse than I expected from the sample circuit given
in the datasheet. This is the schematic, along with the
single-sided pcb layout (in case it's due to poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png
My main scope is out of order and I used my backup 15MHz
single-trace analog scope. It shows narrow spikes of unsteady
amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
level. Moreover, the frequency of about 15 kHz is much lower than
I expected.
The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the
first filter, and only a slight reduction at the output of the
second filter. The load was the LED plus a 470-ohm resistor
(total 24mA).
I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps
of 0.1uF have no discernible effect. The timing cap is a ceramic
disc that shows 465pF on my LCR meter. I wound the inductors with
23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.
What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?
get the correct DC voltage output but was disappointed with the
very dirty output. I know that, in general, simple switched-mode
PSes have poorer performance than linear types, but what I
observed was worse than I expected from the sample circuit given
in the datasheet. This is the schematic, along with the
single-sided pcb layout (in case it's due to poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png
My main scope is out of order and I used my backup 15MHz
single-trace analog scope. It shows narrow spikes of unsteady
amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
level. Moreover, the frequency of about 15 kHz is much lower than
I expected.
The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the
first filter, and only a slight reduction at the output of the
second filter. The load was the LED plus a 470-ohm resistor
(total 24mA).
I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps
of 0.1uF have no discernible effect. The timing cap is a ceramic
disc that shows 465pF on my LCR meter. I wound the inductors with
23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.
What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?